Projections of population demographics in the United States indicate that a significantly higher percentage of the active civilian labor force will reach the age of 55 over the next five to ten years. Given the well-documented evidence of changes in the muscular strength and physiology of older adults, it is clear that the ergonomic design of industrial tasks will have to be reevaluated in the face of potential changes in workers' physical capacity. Previous reports of strength decrements in older muscles have been interpreted to imply concurrent reduction in endurance capability. When performing a task identical to that of a younger worker, older workers will likely be required to perform at relatively higher levels of exertion. Some studies, however, have indicated a higher resistance to fatigue in older muscles when relative effort level is controlled, which may result from a shift towards higher relative proportions of slow- twitch fatigue-resistant fibers. Larger-scale studies have yet to demonstrate the consistency of these observations. Previous investigations have focused on measures of endurance capacity during static exertions, yet typical industrial work is both dynamic and intermittent. The current study will provide much-needed quantitative data on potential differences in endurance during dynamic, intermittent exertions. The three projects outlined in the proposed study will use a variety of experimentally-measured quantitative and subjectively-reported qualitative measures of exertion and fatigue to 1) establish baseline measures of intermittent work capacity in young, healthy workers; 2) compare measures of intermittent work capacity in healthy older workers to those of young adults; and 3) determine the consistency and repeatability of several measures or predictors of work capacity and fatigue. Successful completion of these projects will provide new data for the fundamental characterization of age-related changes in muscle performance and will aid in industrial reevaluation and design of existing workplace tasks. The quantification of intermittent work capacity is an important first step towards effective risk assessment, job evaluation, and reduction of economic and medical costs associated with workplace injuries.